Beverage brewers of this type which serve in particular the function of making filter coffee are known from the Programmubersicht Fruhjahr '88 (Spring 88 Product Overview) of the Braun AG of Russelsheimer Str. 22, 6000 Frankfurt/Main (Date of Issue: April 1988), being listed on page 32 of this publication under Model Nos. KF 45, KF 65 and KF 75.
Coffee makers of this type of construction are in general use nowadays. Their basic structure and their mode of function are described, for example, in DE-U-78 09 444. They comprise a cold water reservoir from which water is initially conducted to a flow-through heater which includes an electrical heating resistor. In this heater, the water is heated to the brewing temperature and subsequently delivered through a riser to a filter cone containing the coffee grinds from which the coffee then flows to an underlying insulated carafe. To maintain the coffee at the desired higher drinking temperature during brewing and also thereafter, the coffee carafe rests on a warming plate which is likewise heated by the heating resistor of the flow-through heater. To enable the heating resistor to serve the dual function of heating the water and keeping the brewed coffee hot within a predetermined temperature range, it is turned off by a temperature-controlled regulator which is thermally coupled to the flow-through heater, generally a bimetallic control element, when its temperature exceeds a predetermined maximum value, being turned on again when its temperature falls below a predetermined minimum value.
The types of coffee maker referred to are equipped with two switches having two switch positions each, the first switch being the "main switch" which, depending on its position, serves to connect or disconnect the coffee maker to or from its electrical supply. The second switch which is referred to as the "mini-brew switch" is switched to its ON position when the user of the coffee maker wishes to brew only small batches of coffee. For this purpose, an electrical circuitry may be used as known from either FIG. 4 of DE-OS 2,839,140 or FIG. 1 of EP-A1-0 186 097.
The circuitry known from DE-OS 2,839,140 operates according to the following principle: With the first switch open, closing of the second switch causes a diode to be connected in the circuit for the voltage supply of the heating resistor between the heating resistor and the source of AC power, so that the heating resistor operates only in the half-wave mode, accordingly delivering only half its rated power. As a result, both the duration of the brewing cycle is extended and the warming plate is heated with less heating power on completion of the brewing cycle. In this manner, the aroma of the coffee grinds is fully utilized also when brewing small batches of coffee because of the prolonged brewing cycle and, in addition, the coffee is kept warm after brewing in a manner preserving its aroma.
In the circuitry known from EP-A1-0 186 097, both the first and second switch have to be closed for brewing small batches. In this arrangement, the regulator which is thermally coupled to the heating resistor is heated by a supplementary heating device when the regulator has opened due to the increase in its temperature, thereby closing again at a moment later than it would do in the absence of the supplementary heating device. This prolongs the intervals during which the power output of the heating resistor is practically zero.
Irrespective of whether the types of coffee maker identified in the foregoing employ the circuitry known from EP-A1-0 186 097 or DE-OS 2,839,140 or another circuitry in which the half-wave mode equally causes the power output from the heating resistor to be halved, the first switch includes a pilot lamp lighting up when the switch is in its ON position. A second pilot lamp capable of indicating the switch position of the mini-brew switch has not been provided so far.
The reason for this is that in all circuitry variants of prior art devices a decoupling of the indicator statuses of the two lamps in dependence on the position of the two switches could only be accomplished by the use of additional "decoupling diodes". In addition to the higher cost incurred by the components themselves, the mounting of further diodes also results in increased manufacturing cost for the corresponding circuits, whereby substantial disadvantages are entailed.